In the preparation of sponge or direct reduced iron utilizing COREX.RTM. gas containing high levels of CO.sub.2 gas (25 to 30%), the gas has heretofore been treated for removal of CO.sub.2 by wet chemical or pressure adsorption methods. It is essential for subsequent direct reduction to remove CO.sub.2 to low levels (2 to 5%) resulting in a high CO/CO.sub.2 ratio (&gt;1 to 1). At these high CO/CO.sub.2 ratios the gas is subject to carbon deposition during reheating especially in the range from 500 to 800.degree. C. by virtue of the following reversible reaction: EQU 2CO=C+CO.sub.2
The subject carbon deposition is detrimental to gas preheater tubes which carburize and deteriorate.